Protective helmet



FIG. 1 is a front perspective view of a protective helmet showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a side view thereof;

FIG. 4 is a back view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a bottom plan view thereof.

The details shown in broken lines form no part of the claimed design. 

The ornamental design for a protective helmet, as shown and described. 